This invention relates in general to stretch blow mold machinery, and to the blow molding of containers. More particularly, this invention relates to an improved blow mold shell having separately defined and independent cooling circuits formed as a part thereof for a container neck forming portion, and a container body sidewall forming portion of the shell, respectively, and to a shell assembly comprised of two substantially identical ones of the shells releasably mated to one another.
A large number of beverages, as well as a variety of food products, are packaged in plastic bottles and/or containers. Plastic has proven to be readily adapted to being formed into a variety of shapes and sizes, and a variety of plastics can be used to form bottles to package beverages, for example, non-carbonated, carbonated, refrigerated or pasteurized beverages, as well as containers for semi-solid food products, to include mayonnaise and peanut butter.
The plastics used to form these containers may include high density polyethylene (xe2x80x9cHDPExe2x80x9d) used primarily for forming milk bottles and for industrial applications, for example forming drums, flasks, and toys; polyvinylchloride (xe2x80x9cPVCxe2x80x9d), the pioneer polymer used in packaging non-carbonated or slightly carbonated beverages; and polyethylene terephthalate (xe2x80x9cPETxe2x80x9d), which is a lightweight transparent plastic material having superior resistance to impact, heat, and pressure, and which is 100% recyclable. PET is among the most commonly used plastics for packaging non-carbonated and carbonated liquids, to include water, fruit juices, soft drinks, cooking oil, household cleaning products, as well as liquids which may have required pasteurization or hot filling.
PET containers are typically formed in one of three types of blow molding operations. These operations include extrusion blow molding, injection blow molding, and lastly stretch blow molding in which a preheated preform, also known as parison, is placed between a pair of releasably mated shell halves, a stretch rod is extended within the preform to stretch it to the desired length, and air is injected into the preform through the stretch rod to force the preform against a mold cavity defined within and as part of the mold shell halves.
Although the use of PET in the stretch blow molding of containers has proven to be extremely successful and has gained widespread acceptance in the marketplace, the problem persists in the ability to adequately and precisely control the temperature of the molds during the blow mold process such that the thinnest allowable container sidewall is molded during the formation of the bottle or container. The greatest expense, approximately 90%, of forming a PET container is not the machinery itself, but rather is the cost of the PET used to produce the container. As modern stretch blow mold machines are capable of attaining production rates beyond 50,000 bottles an hour for standard and flat bottles, for example those bottles capable of being manufactured by the SBO family of stretch blow mold machines manufactured by Sidel, Inc., if the cost of the PET preform can be minimized, this savings, when taking into account the number of bottles produced in an hour, and over the course of a machine""s lifetime, is quite significant.
When blow-molding a PET container it is desirable to mold the upwardly extending neck portion to have a thicker sidewall or cross-section than the elongate sidewall or body portion of the container which extends between the neck and a spaced base portion of the container. It is also desirable that the base portion be molded to have a body section thicker than the sidewall. All that is required of the sidewall is that it posses sufficient structural strength such that it will not rupture during handling or packaging, and will also act as a suitable vapor barrier for sealing the product within the container.
It is known to define a cooling channel within the shell holder or a support plate to which a blow mold shell is fastened so that a cooling fluid is passed through in the cooling channel in the attempt to cool, indirectly, the sidewall of the mold. This, however, has generally proven to be ineffective in attaining the molding of thin sidewall sections. An improved blow mold assembly which formed a cooling channel between the shell holder and the exterior of the mold itself is disclosed in U.S. Pat. No. 3,768,948 to Horberg, Jr. et al. The device of Horberg utilizes the exterior sidewall of the mold shell and a cooperating exterior sidewall of a manifold provided as a part of a shell holder to form a single continuous cooling passageway, which passageway is divided into a number of parallel channels in which fluid is introduced at the neck portion of the mold and allowed to flow along the exterior of the shell until it is discharged approximate the base portion of the mold shell.
An improvement over the device of Horberg, et al. was to provide a cooling circuit defined internally within, and as a part of the mold shell itself. This is illustrated in U.S. Pat. No. 3,601,858 to Blanchard, and in U.S. Pat. No. 5,255,889 to Collette, et al. The problem with these internal cooling circuits, however, is that only a single cooling circuit is provided for cooling both the neck and the sidewall portion of the container, for example, as in Collette, and for also cooling the base of the container as shown in Blanchard. Thus, the ability to separately control the temperature of the mold cavity with respect to the neck and the sidewall portions of the container was not possible. This will result in the sidewall of the container being molded in a thicker section than needed along the body portion of the container, which has the undesirable effect of driving up material costs. Although an improvement over the device of Horberg, et al. for example, these latter two patents still failed to provide a means for adequately cooling the body portion of the container within a unitary shell to allow for the molding of a xe2x80x9cthinxe2x80x9d sidewall.
Several sectional blow mold shell assemblies have been developed in which a plurality of complimentary shaped mold sections may be mated or stacked together for constructing a mold shell (shell half) of a desired shape and size, with each of the individual mold sections being provided with a separate cooling circuit for heat treatment/stress crystallization purposes. Examples of this type of construction are disclosed in U.S. Pat. No. 4,233,022 to Brady, et al.; U.S. Pat. No. 4,701,121 to Jakobsen et al.; U.S. Pat. No. 4,822,543 to lizuka, et al.; U.S. Pat. No. 5,255,889 to Collette, et al.; and U.S. Pat. No. 5,411,698 to Mero, et al.
The problem with using sectional blow mold shell assemblies, however, is the inherent cost of machining the separate mold sections which together cooperate to form the mold shell, to the required degree of precision for defining a continuously shaped molding cavity without unsightly parting or joint lines between each section in the container where these sections adjoin one another, the complexity of the cooling fluid connections to the mold shell sections, the resultant labor costs involved in assembling these molds, and in changing these molds out when differing shaped and sized bottles and/or packaging containers are to be molded.
What is needed, therefore, but seemingly unavailable in the art, is a unitary blow mold shell with an independent cooling circuit for the neck portion of a molding cavity defined within the shell, and a second independent cooling circuit also defined within the shell for cooling the body or sidewall portion of the container as it is molded. There is also a need for such an improved blow mold shell/shell assembly in which the cooling fluid supply line(s), and fluid discharge line(s), respectively, may be placed in direct sealed fluid communication with these cooling channels without otherwise having to be passed through an intermediate support or holder plate to which the mold shell is otherwise mounted. Lastly, there is a need for an improved method of blow molding containers using such a mold shell which allows for the precise control of the thickness of the sidewall of the container during the blow molding process.
The present invention provides an improved blow mold shell, a blow mold shell assembly, and a method of blow molding containers which overcome some of the design deficiencies of the known blow mold shells, shell assemblies, and methods. The invention provides a simple, efficient, and cost effective blow mold shell adapted for use in molding containers in which the temperature of a neck forming portion and a body forming portion, respectively, of a partial molding cavity defined within and by the shell may be separately and precisely controlled such that the sidewall of the container may be molded in as thin a cross-section as possible, or desirable.
The improved blow mold shell of this invention is provided as a unitary blow mold shell having a first end and a spaced second end, with an elongate continuous sidewall extending between its first and second ends, respectively. A partial container molding cavity is defined within an interior surface of the blow mold shell. A first cooling fluid circuit or passageway is defined within the shell with respect to a container neck forming portion of the molding cavity, and a second independent cooling fluid circuit or passageway is defined within the shell with respect to the container body forming portion of the molding cavity, which extends in a lengthwise direction between the first and second ends of the shell.
The blow mold shell assembly of this invention is formed by releasably mating two substantially identical ones of the mold shells together. The shell assembly may also be provided with a separate base member constructed and arranged to be received between the two mold shells as they are closed on one another and about a heated plastic preform during the molding process. The base member defines a base forming portion of the container. A separate third cooling fluid circuit or passageway may be defined within the base member to separately cool the base portion of the container.
Due to the unique construction of the blow mold shell and shell assembly described above, an improved method of blow molding a container is provided. This method is particularly well suited for use on a stretch blow machine for forming a container of the type having a neck, a spaced base, and an elongate body extending between the neck and base thereof. The method includes the steps of positioning a preheated tubular preform at a first end of an elongate blow mold shell assembly, stretching the preform in a lengthwise direction from a first end toward the second end of the shell assembly, and injecting pressurized air into the preform to radially expand the preform against a molding cavity defined within and by the shell assembly.
The method also includes the steps of cooling a neck forming portion of the molding cavity by passing a first cooling fluid through a first cooling fluid circuit defined within the mold with respect to the neck forming portion of the mold so that the neck of the container is molded to have a first wall thickness, and separately cooling a body forming portion of the molding cavity by passing a second cooling fluid through a second independent cooling fluid circuit defined within the mold with respect to the body forming portion so that the sidewall of the container is molded to have a second wall thickness along the body of the container.
It is, therefore, an object of the present invention to provide an improved blow mold shell which is constructed such that a neck forming portion of the mold shell can be cooled separately and independently of a body forming portion of the mold shell.
Yet another object of the present invention is to provide an improved blow mold shell which is simple in design and construction, is rugged and durable in use, and is easy to use.
It is to these objects, as well as to the other objects, features and advantages of the present invention, which will become apparent upon reading the specification, when taken in conjunction with the accompanying drawings, to which this invention is directed.